Yeah, I misspoke.
I was thinking of first strikes being able to wipe out most emplacements before they can be utilized. A nuke nuked is gonna need a replacement if second strike is going to be viable. Even so, anti missile defenses where feared by both sides. If one could be deployed on short notice, if one had been kept secret, they couldn't take the chance that the other side would think itself safe enough for a nuclear exchange. It was no secret that defense shields where, and are in development, even if it was nonviable today, there's no guarantee there wouldn't be a breakthrough tomorrow.